Fluctuating oxygen levels may have accelerated animal evolution, researchers claim

Study shows oxygen levels were far more dynamic.
Nergis Firtina
Fish fossil
Fish fossil


A recent study led by the University of Leeds shows that oxygen levels in the Earth's atmosphere "fluctuated dramatically" one billion years ago, resulting in conditions that may have sped up the evolution of early mammals.

The study was published in Science Advances on October 14. It was also supported by Universities Lyon, Exeter, and UCL.

According to the general operation of scientists, when oxygen first entered the atmosphere about two million years ago, it evolved in three stages, beginning with the Great Oxidation Event two billion years ago, when oxygen first appeared in the atmosphere. Around 400 million years ago, the third stage saw atmospheric oxygen reach current levels.

What is unknown is what transpired during the second stage, the Neoproterozoic Era, which began approximately one billion years ago and lasted for approximately 500 million years and was the era in which the earliest animal species arose.

As said in the release, scientists have been attempting to determine whether there was anything exceptional about the changes in oxygen levels during the Neoproterozoic Era that would have been crucial in the early evolution of animals. So, in light of this statement, we can ask this question: "Did oxygen levels rise suddenly, or did they rise gradually?"

Fluctuating oxygen levels may have accelerated animal evolution, researchers claim
Fossil record of early animals from Mistaken Point Ecological Reserve in Canada.

They managed to produce a record of oxygen levels

Researchers used measurements of the various carbon isotopes present in limestone rocks collected from shallow oceans to attempt to provide an answer to the question.

They were able to compute the amounts of photosynthesis that existed millions of years ago and infer atmospheric oxygen levels based on the isotope ratios of the various forms of carbon that were discovered.

They were able to create a record of the oxygen levels in the atmosphere for the past 1.5 billion years as a result of the computations, which tells us how much oxygen would have been diffusing into the ocean to support early marine life.

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"The early Earth, for the first two billion years of its existence, was anoxic, devoid of atmospheric oxygen. Then oxygen levels started to rise, which is known as the Great Oxidation Event," said Dr. Alex Krause, a biogeochemical modeler who completed his Ph.D. in the School of Earth and Environment at Leeds and was the lead scientist on the project.

“Up until now, scientists had thought that after the Great Oxidation Event, oxygen levels were either low and then shot up just before we see the first animals evolve, or that oxygen levels were high for many millions of years before the animals came along," he added.

“But our study shows oxygen levels were far more dynamic. There was an oscillation between high and low levels of oxygen for a long time before early forms of animal life emerged. We are seeing periods where the ocean environment, where early animals lived, would have had abundant oxygen - and then periods where it does not.”

"Expanded what are known as habitable spaces"

“This periodic change in environmental conditions would have produced evolutionary pressures where some life forms may have become extinct and new ones could emerge,”  Dr. Benjamin Mills, who leads the Earth Evolution Modelling Group at Leeds and supervised the project.

Dr. Mills also added the oxygenated periods expanded what is known as “habitable spaces” – parts of the ocean where oxygen levels would have been high enough to support early animal life forms. 

“When oxygen levels decline, there is severe environmental pressure on some organisms which could drive extinctions. And when the oxygen-rich waters expand, the new space allows the survivors to rise to ecological dominance.  These expanded habitable spaces would have lasted for millions of years, giving plenty of time for ecosystems to develop.”


Mapping the history of atmospheric O2 during the late Precambrian is vital for evaluating potential links to animal evolution. Ancient O2 levels are often inferred from geochemical analyses of marine sediments, leading to the assumption that the Earth experienced a stepwise increase in atmospheric O2 during the Neoproterozoic. However, the nature of this hypothesized oxygenation event remains unknown, with suggestions of a more dynamic O2 history in the oceans and major uncertainty over any direct connection between the marine realm and atmospheric O2. Here, we present a continuous quantitative reconstruction of atmospheric O2 over the past 1.5 billion years using an isotope mass balance approach that combines bulk geochemistry and tectonic recycling rate calculations. We predict that atmospheric O2 levels during the Neoproterozoic oscillated between ~1 and ~50% of the present atmospheric level. We conclude that there was no simple unidirectional rise in atmospheric O2 during the Neoproterozoic, and the first animals evolved against a backdrop of extreme O2 variability.

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